Page images
PDF
EPUB

Attachment IV

Role of Clouds

(FROM IPCC TAR, WORKING GROUP 1; CHAPTER 14)

The role of clouds in the climate system continues to challenge the modelling of climate (e.g., TAR, Section 7.2.2). It is generally accepted that the net effect of clouds on the radiative balance of the planet is negative and has an average magnitude of about 10-20 Wm 2. This balance consists of a short-wave cooling (the albedo effect) of about 40-50 Wm −2 and a long-wave warming of about 30 Wm −2. Unfortunately, the size of the uncertainties in this budget is large when compared to the expected anthropogenic greenhouse forcing. Although we know that the overall net effect of clouds on the radiative balance is slightly negative, we do not know the sign of cloud feedback with respect to the increase of greenhouse gases, and it may vary with the region. In fact, the basic issue of the nature of the future cloud feedback is not clear. Will it remain negative? If the planet warms, then it is plausible that evaporation will increase which probably implies that liquid water content will increase but the volume of clouds may not. What will be the effect and how will the effects be distributed in time and space. Finally, the issue of cloud feedbacks is also coupled to the very difficult issue of indirect aerosol forcing (TAR, Section 5.3).

The importance of clouds was summarized by the 1996 Working Group 1 report of the IPCC: "The single largest uncertainty in determining the climate sensitivity to either natural or anthropogenic changes are clouds and their effects on radiation and their role in the hydrological cycle" (Kattenberg et al., 1996, p345). And yet, the single greatest source of uncertainty in the estimates of the climate sensitivity continues tote clouds (see TAR, Section also 7.2). Since the SAR, there have been a number of improvements in the simulation of both the cloud distribution and in the radiative properties of clouds (TAR, Section 7.2.2). The simulation of cloud distribution has improved as the overall simulation of the atmospheric models has improved. In addition, the cloud sub-component models used in the coupled modes have become more realistic Also, our understanding of the radiative properties of clouds and their effects on climate sensitivity have improved. And yet in Section 7.2.2 we find that, "In spite of these improvements, there has been no apparent narrowing of the uncertainty range associated with cloud feedbacks in current climate change simulations."

Handling the physics and/or the parameterization of clouds in climate models remains a central difficulty. There is a needed for increased observations. J. Mitchell highlighted the challenge in a recent paper at the WCRP Workshop on Cloud Properties and Cloud feedbacks in Large-scale Models where he states that "Reducing the uncertainty in cloud-climate feedbacks is one of the toughest challenges facing atmospheric physicists." 1

Cloud modelling is a particularly challenging scientific problem because it involves processes covering a very wide range of space and time scales. For example, cloud systems extending over thousands of kilometers to cloud droplets and aerosols of microscopic size are all important components of the climate system. The time scales of interest can range from hundreds of years (e.g., future equilibrium climates) to fractions of a second (e.g., droplet collisions). This is not to say that all cloud micro-physics must be included in modelling cloud formation and cloud properties, but the demarcation between what must be included and what can be parameterized remains unclear. Clarifying this demarcation and improving both the resulting phenomenological characterizations and parameterizations will depend critically on improved global observations of clouds2 (see also TAR, Section 2.5.5). Of particular importance are observations of cloud structure and distribution against natural patterns of climate variability (e.g., ENSO). Complementing the broad climatologies will be important observations of cloud ice water and liquid water content, radiative heating and optical depth profiles, and precipitation occurrence and cloud geometry.

The recently approved CloudSat and PICCASO missions, which will fly in formation with the NASA EOS PM (the Aqua Mission), will provide valuable profiles of

1 Mitchell, J. 2000. Modelling cloud-climate feedbacks in predictions of human-induced climate change. In: Workshop on Cloud Processes and Cloud Feedbacks in Large-scale Modes. World Climate Research Programme. WCRP-110; WMO/TD-No. 993. Geneva.

2 Senior, C.A., 1999. Comparison of mechanisms of cloud-climate feedbacks in a GCM. J. Clim., 12, 1480-1489.

cloud ice and liquid content, optical depth, cloud type, and aerosol properties. These observations combined with wider swath radiometric data from EOS PM sensors will provide a rich new source of information about the properties of clouds.3

And yet, this question of cloud feedback remains open, and it is not clear how it will be answered. Given that the current generation of global climate models represents the Earth in terms of grid-points spaced roughly two hundred kilometers apart, many features observed on smaller scales, such as individual cloud systems and cloud geometry, are not explicitly resolved. Without question, the strategy will for attacking the feedback question will involve comparison of model simulations with appropriate observations on global or local scales. The interplay of observation and models, again, will be the key for progress. Mitchell (same reference states this clearly, "Unless there are stronger links between those making observations and those using climate models, then there is little chance of a reduction in the uncertainty in cloud feedback in the next twenty years." This is echoed in the TAR (Section 7.2.2), "A straightforward approach of mode validation is not sufficient to constrain efficiently the models and a more dedicated approach is needed. It should be favored by a larger availability of satellite measurements."

3 Stephens, G., D. Varne, S. Walker. 2000. The CLOUDSAT mission: A new dimension to space-based observations of cloud in the coming millennium. In: Workshop on loud Processes and Cloud Feedbacks in Large-scale Models. World Climate Research Programme. WCRP-110; WMO/TD-No. 993. Geneva.

71-800 D-00--4

Current and Pending Support

(See GPG Section II.D.8 for guidance on information to include on this form.)

The following information should be provided for each investigator and other senior personnel. Failure to provide this information may delay consideration of this proposal.

[blocks in formation]

Mission to Planet Earth: Changes in Biogeochemical Cycles (with J. Aber, B. Rock, D. Skole, C. Vorosmarty, J. Melillo, B. Peterson, W. Emanuel, L. Fisk)

[blocks in formation]

A Web-Based System for Terrestrial Environment Research (with J. Aber, C. Li, C. Vorosmarty, I. Rasool, B. Braswell, F. Rubin)

[blocks in formation]

Modeling the Biogeochemical System of the Terrestrial Amazon: Issues for Sustainability (with C. Vorosmarty,
B. Peterson, S. Pacala, B. Braswell, S. Frolking, C. Li, E. Linder, X. Xiao)

[blocks in formation]

Quantifying the Atmospheric Impacts of Paddy Rice Agriculture in China (with C. Li, S. Frolking, X. Xiao, W. Salas,
R. Sass, R. Harriss)

[blocks in formation]

A Ground-Based Demonstration of Instrumentation for Measuring Wind-Profiles from Space (with J. Ryan, K. Rancourt)

[blocks in formation]

*If this project has previously been funded by another agency, please list and furnish information for immediately preced

ing funding period.

NSF Form 1239 (10/99)

USE ADDITIONAL SHEETS AS NECESSARY

Current and Pending Support

(See GPG Section II.D.8 for guidance on Information to include on this form.)

The following information should be provided for each investigator and other senior personnel. Failure to provide this information may delay consideration of this proposal.

[blocks in formation]

*If this project has previously been funded by another agency, please list and furnish information for immediately preced

ing funding period.

NSF Form 1239 (10/99)

USE ADDITIONAL SHEETS AS NECESSARY

PREPARED STatement of CHARLES F. KENNEL, PH.D.

Good morning, Mr. Chairman and members of the Committee. Thank you for the opportunity to testify. I am Charles Kennel, Director of Scripps Institution of Oceanography, part of the University of California, San Diego. I also serve as Chairman of the National Research Council's Committee on Global Change Research.

Crucial decisions related to environmental changes are made every day by households, insurance companies, water resource managers, agribusiness executives, public health officials, congress, and countless others. These decisions fundamentally affect our nation's health and its economic and environmental vitality. But the National Research Council's (NRC) Committee on Global Change Research (CGCR) concluded that the information necessary to inform these decisions is not always available.

In its recent report titled "The Science of Regional and Global Change: Putting Knowledge to Work," the CGCR found that the United States' observational and modeling capabilities do not adequately serve society's needs for reliable environmental predictions or precise estimation of ongoing changes. This is in part because the federal government does not have mechanisms to establish and provide resources to key research, observational, and technological endeavors that either cross or transcend individual agency responsibilities.

To solve this problem, the National Research Council recommends establishing a high-level governmental authority to define the national priorities related to global and regional environmental research and decision-making. This authority should ensure and direct adequate resources to those priorities. Without such an authority, agencies will continue to fund only those areas that fall within their purview and the resulting patchwork of observing systems and research will not work as an effective decision-support system.

The progress made over the past decade in understanding global environmental change is substantial, as documented in numerous reports of the NRC and in some of the comments of the other witnesses here this morning. This progress has generally been in understanding the effects of single problems in the environment— such as the effect of carbon dioxide on climate or the effect of acid rain on forestswithout considering cumulative effects of multiple factors or the societal context in which the pressures exist. Progress toward sustaining the environmental systems on which life depends is unlikely to be impeded by the individual environmental problems that have occupied the world's attention to date. It is the multiple natural and human factors interacting in a particular location that present the greatest threats and the greatest opportunities. This presents the greatest organizational challenge as well.

A good example of this is the situation in California. That state, which is currently in the midst of an energy crisis, derives up to a third of its electricity from hydropower. Because of the numerous competing uses for water in California, choices must be made as to whether the rain that falls on the state is diverted for agriculture, lake sustenance, drinking water, flood protection, or river flow for recreation, fish habitat maintenance, as well as electricity generation. These choices can be at odds with one another. The changing natural environment compounds the difficulty of these choices, and better information about the changes is needed to inform the decisions.

In 1998 a strong El Niño led to intense rainfall in parts of California. As a result of climate forecasts in the summer of 1997, water resource managers were able to reduce flood damage and power utilities were able take advantage of the high river flows to maximize hydroelectric production and profits. Coastal communities were also able to prepare for the potentially damaging waves that struck the coast in the winter of 1997/1998 and take steps to blunt their force. Local energy companies altered plans to ensure adequate power supplies and to minimize environmental damage to their facilities. There are other success stories from California. But the picture is not all rosy.

The investment in global change research has led to the limited, but valuable capabilities we have now for short-term climate prediction. The range of estimates from NOAA on the annual return on the investment in the El Niño observing network is between 13-26%, compared to a government goal of 7%. This suggests that even imperfect understanding can add significant economic value to society. But there is much more that remains to be done with even greater promise of payoff. We cannot yet predict the start of an El Niño with precision. However, once it begins, we can identify it and assess its progress. We still have very little idea how the strength or frequency of El Niños might change over the course of the next few decades. There are longer-term processes that operate on decadal to century

« PreviousContinue »